Fig. 9 is a diagram of the ordinary induction coil; on a central core is wound a short length of thick wire called the primary, and again over this is wound a greater length of fine insulated copper wire which forms the secondary coil. On sending a low-pressure current from the generator round the thick wire, a much smaller high-tension current is induced in the secondary. A contact breaker is employed to make and break the current, or, as in the early instruments, a commutator may be used to produce the alternations. When used as a transformer the action is reversed, that is, a high-tension current is passed through the primary coil, which is composed of a wire of small sectional area, the high-pressure main connected to the dynamo also being small as compared with the distributing cable leading from the transformer, which, acting as a step-down induction coil, converts the electricity into a safe working pressure. [Illustration: FIG. 9A.] Fig. 9 A shows the arrangement of the two separate and complete circuits. D is the dynamo, P the primary coil, S the secondary, and L the lamps arranged in parallel. It is hardly necessary to go into the technical details of the various improvements which have led up to the modern type of transformers; they are summarised by Mr. Kapp into two classes:— I. Core transformers, one core and two sets of coils. II. Shell transformers, two cores and one set of coils. No. I. are those in which the copper coils are spread over the surface of the iron core enveloping the latter more or less completely; and No. II. in which the core is spread over the surface of the copper coils forming a shell over the winding. [Illustration: FIG. 10.]